/// <remarks>Don't call me unless you are WriteWord</remarks>
void PutWordInCache(TWord word, int bitCount)
{
Contract.Ensures(Contract.OldValue(mCacheBitIndex) == mCacheBitIndex);
// amount to shift word before appending it to mCache bits
int shift = (kWordBitCount - mCacheBitIndex) - bitCount;
TWord word_mask = kBitmaskLUT[bitCount];
word &= word_mask;
word <<= shift;
mCache |= word;
}